This program addresses expression and function of regulatory genes that control development of the immune system. The c-fos oncogene that encodes a DNA-binding nuclear protein is one of such regulatory genes and is implicated to play a role in development and differentiation. To study the function of c-fos gene, we prepared an antisense plasmid that can produce a large amount of c-fos antisense RNA. This antisense plasmid was introduced it into F9 embryonal carcinoma cells. F9 cells that expressed c-fos antisense RNA were unable to induce c-fos mRNA and c-fos protein in response to interferons and phorbol ester and had a reduced basal level of c-fos mRNA. Expression of c-fos gene in these cells could be rescued by cycloheximide treatment, demonstrating that the blockade of c-fos gene expression in the antisense clones is due to inhibition of c-fos message expression by the antisense RNA. Further analyses of the antisense clones showed that the levels of c-fos oncogene is reduced by 5- to 10-fold, indicating a specific linkage between c-fos and c-myc oncogene. Expression of c-fos gene is induced on the day of birth in the mouse; this induction is systemic but transient. We recently found that another immediate early gene, Egr that encodes a Zn finger protein is induced at birth in certain tissues. This, and other reports, indicate that gloval gene activation takes place at birth, which may be responsible for controlling neonatal development. To study the basis of the c-fos gene induction at birth, we searched for a nuclear factor that binds the 5' upstream region of the c-fos gene in a gel mobility shift assay. Nuclear extracts from most adult and fetal tissues that do not express c-fos elicited a slow migrating band, which represents factor binding to the ~20 bp enhancer element: The enhancer controls c-fos induction by serum in tissue culture cells. In extracts obtained at birth, a faster migrating band was detected, which was either absent or very low in the fetal and adult extracts. This neonate-specific band also reprsented a c-fos enhancer binding protein. Additional experiments led us to conclude that the neonatal c-fos enhancer binding activity constitutes a novel factor from the factor present in adult and fetal tissues.